Innate and adaptive immunity are both activated upon vaccination. We have recently demonstrated that the two adaptive immune arms (cellular and humoral immunity) are uniquely induced by different types of myeloid dendritic cell (DC) subsets. In particular, IL-12 induces the development of CD4+ T cells capable of helping B cells through the secretion of IL-21, a cytokine that potently promotes B cell growth, differentiation, and classswitching. These CD4-I- T cells share properties with T follicular helper cells (Tfh), a recently established specialized B cell-helper CD4-f T cell subset. Human Tfh cells can be found in the germinal centers of secondary lymphoid organs and in blood, and are characterized by the expression of the chemokine receptor CXCRS and the secretion of IL-21. Thus, Tfh cells appear to playa fundamental role in the developmentof antibody responses upon vaccination. However, little is known about how vaccines induce Tfh responses and how the induced Tfh cells regulate antibody responses. Our recent studies indicate that Tfh cells from human blood are composed of functionally distinct subsets. The main hypothesis of this project is that the differential mobilization of Tfh subsets dictates the quality and quantity of vaccine-induced humoral immunity. In particular, we hypothesize that a positive vaccine outcome is associated with activation of helper Tfh cells (i.e., Tfh2 and Tfhl7 cells). In contrast, a negative vaccine outcome is associated with either a defect of helper Tfh cell activation or an overactivation of suppressor Tfh cells, i.e., Tfhl cells. In this Project, we will establish the molecular signatures of blood Tfh cells and their functionally different components at baseline, and upon activation following Flu vaccination. The ultimate goal of this Project is to generate tools that will permit high fidelity assessment of in vivo activation of Tfh subsets and prediction of protective responses to vaccines.